“This rise in obesity is due to poor diet and lack of exercise.”

You're writing a blog post about how you think America should solve the problem of having too many overweight people. You just wrote that obesity has increased over the last 20 years, then you off this possible explanation.

In academic writing or when giving speeches, you can use "this" to continue talking about a topic from one sentence to the next:

By the end of the century, the number of hungry people in the world began to increase, rising to 798 million in 2001. This increase in hunger is not too surprising, given the lack of growth in the world grain harvest during this period.